Coal pyrolysis is one of the basic reactions for all coal conversion processes, and coal tar obtained via this reaction is a major raw material of coal chemical industry. At present, the production of coal-based alternative oil/gas and chemicals is mainly based on gasification or catalytic liquefaction of coal. There exists the technical option of pyrolysis that uses the compositional and structural features of coal to produce alternative petroleum, natural gas and chemicals. Theoretical calculation also shows that the direct production of fuel oil and nature gas using volatiles of coal can increase the energy efficiency by 10%-20%, decrease the water consumption by 20%-30% and reduce the CO2 emission by 0.3-0.9 ton per ton coal, as compared to other conventional technical ways. The efficiency of synthetic natural gas (SNG) production by pyrolysis and methanation may be up to 75% and that of fuel oil production by pyrolysis and hydrogenation is possibly high as 65%.
At present, there are two heating methods for carbonization, the internal heating and external heating. The internal heating performs pyrolysis by bringing a high-temperature gas (or solid) heat carrier into the pyrolysis reactor to have it directly contact and interact with coal for pyrolysis. The pyrolysis technologies with internal heating have the advantages of high heat-transfer efficiency, high heating rate, good heating uniformity and so on. However, this kind of heating requires large particles for gas heating carrier in order to ensure good gas permeability in the coal bed, and even for handling coarse fuel particles the dust content in the produced pyrolysis tar are still high, especially when the feedstock contains some small particles. For the direct heating using solid heat carrier such as high-temperature ash, high-temperature char and others, in the reactor these carrier particles directly contact and interact with coal to bring about heat exchange, but in practice there is usually serious entrainment of dust particles by pyrolysis gas, which not only deteriorates the quality of tar but causes also pipeline blockage in some very serious cases. Some studies have been performed to solve these mentioned problems. For example, in the Chinese Patent CN 101818071 A the high-temperature ash, as the heat carrier, is introduced into the middle and lower section of the reactor through multiple feeding pipes, while the raw coal is fed from the top to the interior of the reactor via the interstice among the tubes so that the coal can be simultaneously preheated during its feeding by the hot ash flow inside the tube. Furthermore, the coal bed among the ash-feeding tubes can present certain filtration effect for the formed pyrolysis product when it flows upward and leaves the reactor from the top of the bed. However, this reactor is complicated in structure, and it is particularly prone to have problems of pipeline blockage and non-uniform heating if the treated coal has certain caking property.
The external heating technique refers to that the heat is transferred to material through a heating wall and the material bed is gradually heated from the exterior to the interior. Because no other heating carrier media is introduced, the external heating pyrolysis technology produces tar with relatively low dust content and pyrolysis gas with high heat value. However, when the particle sizes of the raw material or fuel for pyrolysis are small and the material is compactly packed in the reactor, it is very difficult for an external heat source to heat the internal coal bed to lead to non-uniform heating to the material, and there is also a large resistance to the escape of the generated pyrolysis gas and the prolonged residence time of the formed gas inside the bed to cause intensive secondary reactions. These thus cause the problems of low tar yield, high content of heavy oil in tar to lead to low quality of the tar product, low productivity of the technology and the like. On the other hand, the current coal mining technology has to produce a large amount of small-size or powder coal with particle sizes below 20 mm, and it is highly important to effectively use this kind of coal. Patent CN 102212378A devised the use of internals in pyrolysis reactor to enhance the heat transfer in the pyrolysis of carbon-containing materials (fuels) with small particle sizes within a pyrolysis reactor, but this patent did not solve the problems of high escaping resistance and long residence time in the reactor existing for the pyrolysis gas product.
From the preceding analysis one can see that the critical problem existing for the pyrolysis of coal with wide particle size distribution is how to enhance the mass and heat transfer in the process of pyrolysis reaction.